Choosing an Oil-Water Separation Route for Cogeneration Plants: Where Coalescing Technology Fits

Oily wastewater is one of those problems that looks minor on a P&ID and turns into a chronic headache in daily operation. For back-pressure cogeneration plants the stream is surprisingly persistent: turbine and generator cooling water blowdown, leaks and drains from the lubricating oil system, and routine floor washing in the turbine hall. Individually small, continuously generated, and difficult to stabilise.

Start with the oil droplet size, not the equipment catalogue

Most selection mistakes begin with the wrong question. Before comparing equipment, the oil has to be classified by droplet diameter, because each size fraction behaves completely differently in a separator:

• Free oil – droplets above 100 um. Rises quickly under gravity and is easy to skim.
• Dispersed oil – 10 to 100 um. Too small for gravity separation, yet still mechanically separable.
• Emulsified oil – below 10 um. Stabilised by surface charge or surfactants and the main reason a conventional oil trap underperforms.

In a typical cogeneration plant, free oil is the smallest share by mass. The bulk sits in the dispersed and emulsified range, which is exactly where a simple API-type separator stops working.

Three routes, three very different cost structures

Dissolved air flotation (DAF)

DAF saturates water with air under pressure and releases it as a cloud of micro-bubbles that attach to oil droplets and carry them to the surface. The technology is mature and handles large flows well. Its cost, however, is not only capital: coagulants and flocculants are dosed continuously, the float is classified as hazardous waste and needs a disposal route, and the whole train occupies a generous footprint – a real constraint in an existing plant where free space is already committed.

Biological treatment

Biological routes are excellent for biodegradable organic load, but the oil in a power plant is predominantly mineral lubricating and turbine oil. Poor biodegradability combined with microbial inhibition means biology only works after effective de-emulsification and oil removal upstream, which lengthens the train and adds operating complexity.

Coalescing separation

This is the physical route, and the one that generally suits cogeneration duty best. Inside a coalescing oil-water separator, oleophilic and hydrophobic fibre media capture micron-sized droplets as water passes through. Droplets attach, merge with their neighbours, and grow. Once a droplet reaches roughly millimetre scale, buoyancy overcomes the adhesion force, it detaches, and it rises to the oil collection chamber at the top while clarified water leaves from the bottom.

SINOKLE's KHC coalescing oil-water separator is a representative product of this approach: a single unit integrates coalescing separation and automatic oil discharge, occupying a footprint equivalent to only a small horizontal storage tank.

The operational consequences matter more than the mechanism: no chemical dosing, no heating, no hazardous sludge stream – it is purely physical separation.

What the numbers looked like on a real 5 m3/h unit

A back-pressure cogeneration plant in Shaanxi commissioned a 5 m3/h train in September 2022. The feed is a typical lubricating oil and turbine oil emulsion with oil concentration varying in the low-to-medium band. A primary filter upstream intercepts large suspended solids as pre-treatment, and the KHC coalescing oil-water separator performs the core separation downstream.

• Effluent oil content after commissioning: stably <5 mg/L
• Reference limits: 10 mg/L under the national first-class standard, 5 mg/L under the strictest local standard
• Continuous operation for three years with no media compaction or measurable efficiency loss

In other words, the unit meets the tightest local limit directly, without a polishing stage.

When coalescing separation is the right answer – and when it is not

Coalescing separation is usually the best overall value where most of the following apply: flow in the 1–20 m3/h range, oil concentration at or below about 500 mg/L, oil type dominated by mineral oils such as lubricating and turbine oil, tight effluent stability requirements, and limited available plot space.

It is not universal. Very high oil concentrations, or streams loaded with heavy oil and tar, need reinforced pre-treatment ahead of the coalescer, or a different process combination altogether. Selection is always scenario-specific; the value of a pilot or a jar test before committing to a capital purchase is hard to overstate.